The realization that losses in and material dispersion of silica based optical fibers may be minimized by using wavelengths within the region between 1.0 and 1.6 microns (.mu.m) has stimulated interest in light sources and photodetectors capable of operating within that wavelength region. Special interest has been directed toward the wavelength region from approximately 1.0 to 1.6 .mu.m because that appears, at present, to be the optimum wavelength region with respect to minimum light losses.
A photodetector is an essential component of an optical communication system and much effort has been directed toward developing structures and materials for such structures. Due to the favorable size of the bandgaps, interest has centered on devices, both light sources and photodetectors, using III-V elements. Devices using binary semiconductors, such as GaSb; ternary semiconductors, such as InGaAs, and GaAlSb; and quaternary semiconductors, such as InGaAsP; have been constructed with III-V elements. For example, avalanche photodiodes and p-i-n photodiodes with different structures and combinations of the III-V elements previously mentioned have been considered for use. The photodetector ultimately selected should satisfy several requirements. It should have good quantum efficiency, low dark current characteristics, and relatively low capacitance.
One approach to photodetectors operating within the wavelength region between 1.0 .mu.m and 1.6 .mu.m uses InGaAs grown on InP substrates. Different embodiments of this approach are described in Applied Physics Letters, 33, pp. 640-642, Oct. 1, 1978 and Electronics Letters, 16, pp. 155-156, 1980. The former article described a homojunction avalanche photodiode which had a reverse current of 2 nA at half the breakdown voltage. This current corresponded to a dark current density of 4.times.10.sup.-6 A/cm.sup.2. The latter article described a p-i-n photodiode which was back illuminated and had a dark current of 2 to 5 nA at a reverse bias of 10 volts.
Another approach to photodetectors operating within the same wavelength region is described in Applied Physics Letters, 35, pp. 251-253, Aug. 1, 1979. This article described an InGaAsP heterostructure avalanche photodiode having layers of n-type InGaAsP and n-type InP sequentially grown on an n+-type InP substrate. A p+-type InP region is formed in the InP layer by cadmium diffusion. The resulting photodiode operates with an avalanche junction in the wider bandgap material, InP, near the narrow gap material, InGaAsP, with the latter material absorbing the light. The device is stated to have good quantum efficiency and avalanche gain when operated at a relatively high reverse bias. Quantum efficiencies of greater than 60 percent and an avalanche gain of 3000 were reported. However, a valence band barrier of approximately 0.4 volts blocks the diffusion of holes producted by light absorption in the quaternary layer. The holes surmount the barrier only when there is a large electric field near the heterojunction which can accelerate holes. Further, when operating in the avalanche region, tunneling occurs and the dark current rises. Due to the presence of large tunneling currents which cause a significant amount of detector noise, it may be difficult to make low noise avalanche photodetectors from III-V materials.